1. Technical Field
The present disclosure relates to electrosurgical devices and, more particularly, to systems for thermal-feedback-controlled rate of fluid flow to a fluid-cooled antenna assembly and methods of directing energy to tissue using the same.
2. Discussion of Related Art
Treatment of certain diseases requires the destruction of malignant tissue growths, e.g., tumors. Electromagnetic radiation can be used to heat and destroy tumor cells. Treatment may involve inserting ablation probes into tissues where cancerous tumors have been identified. Once the probes are positioned, electromagnetic energy is passed through the probes into surrounding tissue.
In the treatment of diseases such as cancer, certain types of tumor cells have been found to denature at elevated temperatures that are slightly lower than temperatures normally injurious to healthy cells. Known treatment methods, such as hyperthermia therapy, heat diseased cells to temperatures above 41° C. while maintaining adjacent healthy cells below the temperature at which irreversible cell destruction occurs. These methods involve applying electromagnetic radiation to heat, ablate and/or coagulate tissue. Microwave energy is sometimes utilized to perform these methods. Other procedures utilizing electromagnetic radiation to heat tissue also include coagulation, cutting and/or ablation of tissue.
Electrosurgical devices utilizing electromagnetic radiation have been developed for a variety of uses and applications. A number of devices are available that can be used to provide high bursts of energy for short periods of time to achieve cutting and coagulative effects on various tissues. There are a number of different types of apparatus that can be used to perform ablation procedures. Typically, microwave apparatus for use in ablation procedures include a microwave generator that functions as an energy source, and a microwave surgical instrument (e.g., microwave ablation probe) having an antenna assembly for directing the energy to the target tissue. The microwave generator and surgical instrument are typically operatively-coupled by a cable assembly having a plurality of conductors for transmitting microwave energy from the generator to the instrument, and for communicating control, feedback and identification signals between the instrument and the generator.
There are several types of microwave probes in use, e.g., monopole, dipole and helical, which may be used in tissue ablation applications. In monopole and dipole antenna assemblies, microwave energy generally radiates perpendicularly away from the axis of the conductor. Monopole antenna assemblies typically include a single, elongated conductor. A typical dipole antenna assembly includes two elongated conductors, which are linearly aligned and positioned end-to-end relative to one another with an electrical insulator placed therebetween. Helical antenna assemblies include a helically-shaped conductor that can be formed in various configurations. The main modes of operation of a helical antenna assembly are normal mode (broadside), in which the field radiated by the helix is maximum in a perpendicular plane to the helix axis, and axial mode (end fire), in which maximum radiation is along the helix axis.
A microwave transmission line typically includes a thin inner conductor that extends along the longitudinal axis of the transmission line and is surrounded by a dielectric material and is further surrounded by an outer conductor around the dielectric material such that the outer conductor also extends along the transmission line axis. In one variation of an antenna, a waveguiding structure, such as a length of transmission line or coaxial cable, is provided with a plurality of openings through which energy “leaks” or radiates away from the guiding structure. This type of construction is typically referred to as a “leaky coaxial” or “leaky wave” antenna.
Because of the small temperature difference between the temperature required for denaturing malignant cells and the temperature normally injurious to healthy cells, a known heating pattern and precise temperature control is needed to lead to more predictable temperature distribution to eradicate the tumor cells while minimizing the damage to surrounding normal tissue. Excessive temperatures can cause adverse tissue effects. During the course of heating, tissue in an overly-heated area may become desiccated and charred. As tissue temperature increases to 100° C., tissue will lose water content due to evaporation or by the diffusion of liquid water from treated cells, and the tissue becomes desiccated. This desiccation of the tissue changes the electrical and other material properties of the tissue, and may impede treatment. For example, as the tissue is desiccated, the electrical resistance of the tissue increases, making it increasingly more difficult to supply power to the tissue. Desiccated tissue may also adhere to the device, hindering delivery of power. At tissue temperatures in excess of 100° C., the solid contents of the tissue begin to char. Like desiccated tissue, charred tissue is relatively high in resistance to current and may impede treatment.
Microwave ablation probes may utilize fluid circulation to cool thermally-active components and dielectrically load the antenna radiating section. During operation of a microwave ablation device, if proper cooling is not maintained, e.g., flow of coolant fluid is interrupted or otherwise insufficient to cool device components sensitive to thermal failure, the ablation device may be susceptible to rapid failures due to the heat generated from the increased reflected power. In such cases, the time to failure is dependent on the power delivered to the antenna assembly and the duration and degree to which coolant flow is reduced or interrupted.
Cooling the ablation probe may enhance the overall heating pattern of the antenna, prevent damage to the antenna and prevent harm to the clinician or patient. During some procedures, the amount of cooling may not be sufficient to prevent excessive heating and resultant adverse tissue effects. Some systems for cooling an ablation device may allow the ablation device to be over-cooled, such as when the device is operating at low power settings. Over-cooling may prevent proper treatment or otherwise impede device tissue effect by removing thermal energy from the targeted ablation site.